For example, turbine components, such as turbine blades for instance, are produced inter alia from superalloys. Superalloys are nickel- or cobalt-based alloys and typically comprise chromium, titanium, tantalum, aluminum, tungsten and further elements with outstanding resistance to high temperatures and at the same time high strength. An example of a gas turbine blade produced from a superalloy can be taken from U.S. Pat. No. 5,611,670.
The superalloy material has a crystalline structure. As in other crystalline structures, crystal defects may occur in superalloy material. A typical crystal defect is that of grain boundaries, i.e. boundaries between different regions of material which, though they have the same crystalline structure, have a different crystallographic orientation. In directionally solidified structural components, the grain boundaries in the material substantially extend only in one direction, for example in the radial direction of the turbine blade. In monocrystalline materials, on the other hand, there are substantially no grain boundaries.
Structural components of a superalloy such as the said turbine blades are cast and subjected to further working steps after casting. The further working steps comprise a heat treatment to homogenize the material, a so-called solutioning heat treatment, which results inter alia in an increase in the strength of the turbine blade. In addition, the cast turbine blades are also subjected to mechanical working processes. The mechanical working processes thereby cause stresses in the material of the structural components, which may lead during the solutioning heat treatment to the formation of new grains (recrystallization) and consequently to grain boundaries. These grains make the material properties of monocrystalline or directionally solidified structural components deteriorate and, as a result, have an adverse influence on the lifetime of the component. In order to avoid this, the freshly cast structural components are subjected to the heat treatment under an inert gas atmosphere or under a vacuum immediately after the casting process and partial removal of the casting mold. Only after that are the mechanical working processes performed on the component.
If new structural components have to be reworked or structural components that are already in operation have to be repaired, for example by welding or soldering, this may necessitate a renewed heat treatment at the temperature of the solutioning heat treatment or close to this temperature. On account of the stresses that are still present from the mechanical working after the casting process, this may lead to the formation of recrystallized grains. On the other hand, a solutioning heat treatment can increase the service life of the structural components, in particular in the case of components which have already been in operation for a certain amount of time.